"It is because the contemporary alternatives seem so one-sided and are not more evidently solutions to the problems which Thomas faced, and partly solved, that we return to him and to the tradition of theology and philosophy in which his Summa Theologiae appears: theology as the science of the first principle and this as the total knowledge of reality in its unity." -- Wayne J. Hankey, God in Himself (Oxford University Press, 1987), p.159.

Just as nature does not fail man in necessaries, although it has not provided him with weapons and clothing, as it provided other animals, because it gave him reason and hands, with which he is able to get these things for himself; so neither did it fail man in things necessary, although it did not give him a definite principle by which he would attain Happiness, since this is impossible [because of what Happiness, by its nature, is]. But it did give him free choice, with which he can turn to God, that He may make him happy [by God's friendship with him]: "For what we do by means of our friends, is done, in a sense, by ourselves" (Ethic. iii, 3).

The natural knowledge of every creature is in keeping with the mode of his substance; thus it is said of the intelligence (De Causis; Prop. viii) that "it knows things that are above it, and things that are below it, according to the mode of its substance." But every cognition that is according to the mode of created substance, falls short of the vision of the Divine Essence, which infinitely surpasses all created substance. Consequently neither man, nor any creature, can attain final Happiness by its natural powers.

For not everything, that can cause the disposition of matter, can produce the final perfection. Now the imperfect operation, which is subject to man's natural power, is not of the same species as that perfect operation which is man's happiness, since operation takes its species from its object.

The nature that can attain perfect good, although it needs help from without in order to attain it, is of more noble condition than a nature which cannot attain perfect good, but attains some imperfect good, although it need no help from without in order to attain it, as the Philosopher says (De Coel. ii, 12).

Thus he is better disposed to health who can attain perfect health, albeit by means of medicine, than he who can attain but imperfect health, without the help of medicine. And therefore the rational creature, which can attain the perfect good of happiness, but needs the Divine assistance for the purpose, is more perfect than the irrational creature, which is not capable of attaining this good, but attains some imperfect good by its natural powers.

Imperfect happiness that can be had in this life, can be acquired by man by his natural powers, in the same way as virtue, in whose operation it consists; on this point we shall speak further on (q63).